Ultrasound of the infant hip.

dislocations have negative or equivocal signs at birth (Wilkinson, 1985). Since the introduction of screening the incidence of CDH has remained largely unchanged for a number of complex reasons, not all of which are understood (Catford et al., 1982). CDH encompasses a spectrum of abnormalities ranging from minor dysplasia to frank dislocation. On clinical examination between 15 and 20 infants per 1000 live births will have unstable


INTRODUCTION
Screening of all neonates for congenital dislocation of the hip (CDH) within twenty-four hours of birth has become established clinical practice. The diagnosis of CDH can however, be difficult, and many children with persistent or established dislocations have negative or equivocal signs at birth (Wilkinson, 1985). Since the introduction of screening the incidence of CDH has remained largely unchanged for a number of complex reasons, not all of which are understood (Catford et al., 1982).
CDH encompasses a spectrum of abnormalities ranging from minor dysplasia to frank dislocation. On clinical examination between 15 and 20 infants per 1000 live births will have unstable hips. Most of these resolve without specific treatment with only ten percent going on to eventually dislocate, and a further ten percent showing signs of dysplasia (Standing Medical Advisory Committee DHSS, 1986). Difficulty in knowing which hip will resolve and which requires treatment has led many authorities to advocate immediate splinting of all clinically unstable hips (Dunn et al., 1985). Early splinting in abduction, however, may occasionally result in avascular necrosis of the femoral head.
This, and the uncertainty about the natural history of the disease, have resulted in controversy over the detection and optimal management of CDH (Dunn et al., 1985;Leek, 1986;Wilkinson 1985). Ultrasound has a potentially valuable role in contributing to the diagnosis and management of CDH. It is an accurate and sensitive test for identifying those hips that require treatment (Graf. 1984;Clarke, 1986) and for detecting structural abnormalities in hips that are clinically silent (Berman & Klenerman, 1986;Clarke et al., 1989). It was however, initially received without enthusiasm as it was a difficult and time consuming examination to perform with the static B-scanning equipment on which it was pioneered (Graf, 1980). The development of high quality real time ultrasound has greatly simplified the examination and made it a technique potentially available in all radiology departments. Ultrasound offers a number of immediate advantages over other imaging investigations. Plain film radiology is relatively unhelpful in the immature pelvis as so little of the hip is ossified. With ultrasound both bony and non-osseous structures are shown allowing the femoral head and its surrounding structures to be demonstrated. It is a dynamic investigation which can be repeated at appropriate intervals to follow the natural history of a questionable hip without submitting, the child to ionizing radiation. The effects of splinting can also be followed to ensure that the femoral head remains appropriately located. If there is failure to maintain concentric reduction, splinting can be abandoned early, so reducing the risk of avascular necrosis (Clarke et al., 1989).

TECHNIQUE
Most high quality commercially available ultrasound machines are suitable for imaging the infant hip. A 5 MHz near focus transducer is a satisfactory compromise between resolving power and depth of penetration. For children less than three months of age, a 7 MHz transducer will provide higher resolution without undue loss of penetration. A linear array probe is preferred as it is easier to use and has a wider field of view for objects near to the skin than a sector scan. A combination of two views, using easily identifiable landmarks, are used to demonstrate the anatomical configuration of the acetabulum and detect displacement of the femoral head (Harke et al., 1984; Clarke et al., 1985). The first view is obtained with the hip in the neutral position and the scan section in the transverse or axial plane (transverse-neutral view). The transducer is positioned on the upper femur and advanced cephalad until the femoral head is shown (Fig. 1). The second view is performed with the hip flexed to ninety degrees and the scan section in the coronal plane (coronal-flexion view). The transducer is positioned on the upper femur and advanced posteriorly along the shaft until the femoral head is again shown (Fig. 2 (Fig. 3).
In the coronal-flexion view the bony acetabulum is surmounted by a triangular cartilaginous roof which extends over the femoral head and to which is attached the triangular labrum (Fig. 2). The labrum is composed of echogenic fibrocartilage. In the caudal portion of the acetabulum the strongly echogenic band of the legamentum teres can frequently be seen. This view enables the configuration of the acetabulum, the degree of coverage, and the position of the femoral head to be assessed (Figs. 2&4). The ossific nucleus is visible radiographically between two and six months of age in a girl, and three to seven months in a boy. Ultrasound will demonstrate the nucleus 14 to 21 days earlier but because it casts an acoustic shadow only the lateral portion is shown the 'half-moon phenomenon' of Graf (Fig.  5). Delayed and asymmetric development of the ossific nucleus is frequently seen in dysplastic hips. Acoustic shadowing from an ossific nucleus greater than 10 mm in diameter will obscure the triradiate cartilage and not allow the exact position of the femoral head to be determined so limiting the value of ultrasound in children more than a year old (Clarke, 1985). Prior to ossification, small blood vessels in the femoral head can be recognized as echogenic, non-shadowing speckles (Fig. 6). Example of normal and pathological hip examinations are illustrated in figures 1 to 6. CONCLUSION Various techniqes for performing ultrasound of the hip have been described. Some of these rely on a single coronal view and complex measurements to classify the hip (Graf, 1984). As with many procedures, familiarity with the range of normal and care in obtaining reproducible standard views, are at least as important as the particular method chosen.
For the moment, ultrasound of the infant hip is confined to assessing questionable abnormalities detected on clinical examination during the first year of life. The dynamic nature of the examination allows the location of the femoral head to be determined in neutral and passive flexion and abduction, as well as under stress using and Barlow and Ortolani manoeuvers.
The anatomical configuration of the bony and cartilaginous portions of the acetabulum are also well shown. Serial studies allow close monitoring of the subsequent development of the femoral head and acetabulum in problem hips without submitting the child to ionizing radiation. Ultrasound has the potential of resolving the difficulties concerning the natural history and management of CDH but before it can be used as a screening examination further assessment is required in large controlled trials with adequate long term follow-up (Clarke et al., 1989;Scott, 1989).  A-anterior, L-lateral, p-psoas major muscle, 1-echogenic osified portion of pubis, 2-echogenic ossified portion of ischium, curved arrowcartilaginous femoral head, open arrow-greater trochanter, large straight arrow-hypoechoic triradiate cartilage with medial through transmission, arrow head-hypoechoic pubic carbilaginous portion of the acetabulum, small arrow-echogenic ligamentum teres.           Coronal-flexion view. The femoral head is located within a well developed acetabulum. The ossific nucleus (large crossed arrow) appears as a half-moon casting an acoustic shadow medially (small arrows). Coronal-flexion view. The femoral head is located within a well developed acetabulum. The ossific nucleus (large crossed arrow) appears as a half-moon casting an acoustic shadow medially (small arrows).

Figure 6
Coronal-flexion view. Normal hip with echogenic non-shadowing blood vessels (curved arrow) in the region of the nucleus prior to its ossification. Figure 6 Coronal-flexion view. Normal hip with echogenic non-shadowing blood vessels (curved arrow) in the region of the nucleus prior to its ossification. 76